1. Field of the Invention
The present invention relates to an evaporated fuel treatment device of an engine.
2. Description of the Related Art
Known in the art is an internal combustion engine provided with a canister for temporarily storing evaporated fuel generated in a fuel tank and a purge control valve for controlling the amount of purge of the fuel vapor to be purged from the canister to the inside of an intake passage, performing feedback control on the air-fuel ratio so that the air-fuel ratio becomes the target air-fuel ratio, calculating the concentration of fuel vapor per unit purge rate from the amount of deviation of the air-fuel ratio from the target air-fuel ratio, finding the current concentration of fuel vapor from the concentration of fuel vapor per unit purge rate and the current purge rate, and correcting the amount of fuel injection based on the concentration of fuel vapor (see Japanese Unexamined Patent Publication (Kokai) No. 7-293362.
That is, when trying to purge fuel absorbed in a canister into the intake passage, the amount of fuel vapor which is purged is proportional to the purge rate, therefore the concentration of the fuel vapor in the intake air is also proportional to the purge rate. That is, the concentration of fuel vapor in the intake air increases in proportion to an increase of the purge rate. Accordingly, if the concentration of fuel vapor per unit purge rate is found, it is possible to find the concentration of fuel vapor at that time, no matter how the purge rate changes, by multiplying the purge rate with the concentration of fuel vapor per unit purge rate. Consequently, even in the above internal combustion engine, the concentration of the fuel vapor per unit purge rate is found from the amount of deviation of the air-fuel ratio from the target air-fuel ratio.
By finding the concentration of fuel vapor per unit purge rate in this way, it is possible to accurately find the concentration of fuel vapor at that time even if the purge rate changes so long as the concentration of fuel vapor is proportional to the purge rate, therefore if the amount of fuel injection is corrected based on the concentration of fuel vapor, it would become possible to maintain the air-fuel ratio at the target air-fuel ratio no matter how the engine operating state changes.
The actual concentration of fuel vapor however includes a portion which changes proportional to the purge rate as explained above and a portion which is not proportional to the purge rate. If there is a portion which is not proportional to the purge rate in this way, the product of the concentration of the fuel vapor per unit purge rate and the purge rate will no longer accurately express the concentration of the fuel vapor. If the amount of fuel injection is corrected based on this concentration of fuel vapor, the problem will arise of the air-fuel ratio deviating from the target air-fuel ratio.